Oh wow... you might have saved me a lot of work. I too have a Ghost Heart/chanter. I posted this issue in several game forums, and was told be experienced players that this was intended, otherwise it would be over powered to have multiple summons out at once. At 30+ hours into the game, it really makes my character build concept pointless, and I was on the brink of trying anything other than have to restart from scratch.
Hopefully the devs can get us the details (and fix?) quckly, as id love to start playing again.